This invention relates to coupling devices for coupling and decoupling a driving shaft from a prime mover to a driven shaft from which useful work may be extracted. In particular, this invention is directed to a means for decoupling such shafts such that when once decoupled they can not be recoupled until the driving shaft has come to an almost complete stop.
Such a coupling is used, for example, in the operation of an auxiliary power unit for modern jet aircraft. In this application the output shaft of the gas turbine of the auxiliary power unit is connected to drive accessories for the aircraft such as the electric power generators. In addition, the APU output shaft is selectively coupled by means of the coupling of this invention, to an air compressor input shaft. The start up procedure for the aircraft would be as follows: The APU is started by means of stored power such as a battery, with the coupling to the air compressor engaged. When the APU is at operating speed the output of the compressor is used to start the main propulsion engines. When these engines are operating, the compressor is decoupled and the APU used for accessory drive and any compressed air required by either aircraft systems is supplied by bleed air from the propulsion engines. Friction clutches are generally rather bulky if the transmitted load is large and, further, in the decoupling and coupling action wear material is deposited in the assembly where it is detrimental to the operation of the device. Further, such clutches are difficult to maintain in balance for rotation at high operational speeds. In the case of toothed or sprag clutches, it is difficult to decouple such devices while under load and such couplings generally have high point loading problems and are complex and costly to manufacture.
The present invention overcomes these disadvantages by providing a decoupling mechanism which is readily decoupled while under full load and cannot be recoupled until shafts are at near standstill condition. Further it gives the decoupling ability of a friction clutch in a much smaller precise package that can be rotated and balanced at high speeds without depositing wear material in the mechanism.
These functions are accomplished by the use of a simple hydraulic cylinder which when activated rotates a collar having pockets to receive the connecting rollers of the coupler. When these pockets are aligned with the rollers of the coupler, the centrifugal force of the rotating shaft forces the rollers into the pockets and out of engagement with the driven shaft. Since the rollers are maintained out of engagement by centrifugal force, they will not recouple until this force has been removed by stopping rotation of the shaft.